1. Field of the Invention
This invention relates to an intake system for an internal combustion engine and more particularly to an engine intake system making use of inertial supercharging.
2. Description of the Prior Arts
A well-known intake system making use of inertial supercharging is a supercharging system for enhancing volumetric efficiency, by using both the inertial effect of air flowing through an intake passage immediately before the end of a suction stroke and the pressure wave effect obtained by synchronizing the introduction of pressure waves in the intake passage into an engine cylinder and the closing timing of an intake valve; this is an effective means for augmenting an average effective pressure of an engine.
In order to attain the inertial effect, however, a predetermined or higher engine r.p.m. is required, depending upon the length of the intake passage. At low engine speed therefore, the flow velocity of the air in the intake pipe is so low that the inertial effect also is low, and ineffective. In order to attain the pressure wave effect, on the other hand, the arrival timing of the pressure waves having a propagating sound velocity at the engine cylinder and the closing timing of the intake valve have to be synchronized at a specified engine speed. At an engine speed other than the specified one, negative pressure waves become timed with the closing of the intake valve so that the pressure wave effect generally becomes negative and troublesome.
Thus, generally speaking an automotive engine makes use of the pressure wave effect at high speed, when the inertial effect is high, with a view to augmenting its output. This in turn means that low-speed torque necessary for starting the engine cannot be increased.
In either a petrol or a diesel engine, on the other hand, vortexes of the air in the cylinder are desirable for improving combustion efficiency and for removing the exhaust gas. In a known intake system, the intake passage of the engine is shaped to establish intense vortexes only at a specified engine speed. As a result, the vortexes are intense at a higher engine speed but mild at a lower engine speed, so that vortexes of the required intensity are generated only at an engine speed within a specified, narrow range. Thus combustion will be less effective if that specified range is not covered.
On the other hand, a turbo-charged engine has low supercharging capacity at low speed because of characteristics of the turbo-charger, which is ineffective at a high supercharging pressure so that it cannot generate good low-speed torque. Therefore some turbo-charged engines are designed to use the aforementioned pressure wave effect as to increase torque within a narrow range at low speed. Outside that speed range, however, the pressure wave effect exerts an adverse effect, with the disadvantage that it cannot enhance a low-speed torque, i.e. the torque required for starting the vehicle.